It depends on how good a ground you want. If you only want to keep the house from burning down, a connection to the metal box and conduit will work. On the other hand, if you're trying to protect computer files and sensitive electronic equipment, you will want an actual ground wire that connects to the panel ground bus. The problem is static electricity. It doesn't take much to confuse a computer, especially around a thunderstorm. I used to install and service computers and have heard some professional electricians say "a ground is a ground". Wrong. Many times I've seen computer problems cleared up by running an isolated ground wire to the bus, even when there is already a ground wire under a screw on the side of the breaker box or elsewhere. The ground wire should be as heavy or heavier than the hot wire for that circuit.
Today's computers are not as sensitive as they used to be, but it doesn't take even a direct hit by lightning to take out your tv, vcr, cordless phone, answering machine, etc. A good ground is not a cure-all but it will help.